1. Field of the Invention
The present invention relates to a laminated coil and, more specifically, to a laminated coil having an excellent direct current (DC) superimposition characteristic.
2. Description of the Related Art
A laminated coil is produced by stacking magnetic sheets each composed of ferrite or other suitable magnetic material and provided with a coil conductor composed primarily of Ag. Such a laminated coil is used in various circuits. In the laminated coil, the effective magnetic permeability is increased and a high inductance value is obtained because a closed magnetic path is produced by the magnetic field that is generated by an electrical current flowing through the coil conductors. The laminated coil is also advantageous in that loss caused by the conductor resistance is small because the conductor patterns are primarily composed of Ag. Thus, the laminated coil is used as a choke coil for a switching power supply to which a high current is applied.
For coil elements, the relationship between the current value applied to the coil conductors and the inductance value is represented as a DC superimposition characteristic. For a laminated coil having a closed magnetic path, there is a problem in that the desired choke coil characteristic cannot be obtained because the inductance value quickly decreases when the current exceeds a predetermined value. This degradation of the DC superimposition characteristic is caused by magnetic saturation in the magnetic body generated because the laminated coil produces a closed magnetic path.
To solve the above-identified problem, the laminated coil described in Japanese Unexamined Patent Application Publication No. 2001-44036 includes non-magnetic body layers that are provided inside the laminated coil composed of ferromagnetic layers. With the structure described in Japanese Unexamined Patent Application Publication No. 2001-44036, a closed magnetic path is less likely to be produced inside the magnetic body because the magnetic flux from the non-magnetic body layers leak outside the laminated coil. Thus, magnetic saturation is not likely to occur, and the DC superimposition characteristic is improved.
However, according to the structure of Japanese Unexamined Patent Application Publication No. 2001-44036, the amount of magnetic flux that leaks from the non-magnetic body layers is limited because the coil conductors provided on the non-magnetic body layers and the coil conductors provided on the ferromagnetic layers have the same shape and the same number of coil turns. Therefore, when the value of the electric current flowing through the coil conductors is increased, the DC superimposition characteristic is likely to deteriorate.